Removing heat as efficiently and immediately as possible is required for an element which generates a large heat during operation, such as a power semiconductor element used for drive control of an electric vehicle, a hybrid vehicle, or the like, a light emitting element such as a laser diode, a control element for a base station for mobile phones and the like, an image display element for a plasma display panel or the like, or a microprocessor unit for a computer.
This is because, if the generated heat is not removed efficiently and immediately, the element itself may overheat and malfunction (thermally run away), or the element, a package which covers the element, or the like may be broken.
In recent years, with enhancement in performance and output of various apparatuses using these elements, a shift from elements commonly used at present, such as silicon (Si)-based, gallium arsenide (GaAs)-based, and indium phosphide (InP)-based elements, to elements such as silicon carbide (SiC)-based and gallium nitride (GaN)-based elements is under consideration.
In that case, it is possible to increase the operable temperature of an element, for example, from around 120° C. for the silicon-based element and the like to around 200° C. for the silicon carbide-based element and the like, and it is considered that malfunction, breakage, and the like due to overheating are less likely to occur than before.
However, even when these elements are used, it is still required to remove heat from these elements as efficiently and immediately as possible.
In order to remove heat from an element efficiently and immediately, it is common to use a heat spreader formed in the shape of, for example, a plate as a whole, and having one surface (element mounting surface) to which an element is solder-bonded directly or with a ceramic substrate or the like interposed therebetween, and an opposite surface (heat dissipating surface) to which a cooling mechanism such as a cooler is connected.
Conventionally, heat spreaders formed integrally as a whole using a metal such as copper (Cu) or aluminum (Al) or an alloy have been used. Recently, however, using a heat spreader including a copper-molybdenum (Cu—Mo) layer made of a Cu—Mo composite material, which has a thermal expansion coefficient close to that of an element such as the Si-based, GaAs-based, InP-based, SiC-based, and GaN-based element described above, that of a ceramic substrate made of aluminium nitride (AlN), aluminium oxide (Al2O3), or silicon nitride (Si3N4), or the like, is under consideration.
By providing the Cu—Mo layer, the thermal expansion coefficient of the heat spreader can be decreased to be less than that of a conventional heat spreader entirely made of a metal or an alloy, and can be brought closer to the thermal expansion coefficient of the element or the ceramic substrate as much as possible. That is, matching between the thermal expansion coefficients thereof can be achieved.
This can prevent an excessive stress from being applied to the element or the ceramic substrate (hereinafter may be abbreviated as an “element or the like”) based on the difference in thermal expansion coefficient, under a thermal load environment such as a hot-cold cycle which repeats heat generation due to operation of the element and cooling after stop thereof, and breaking the element or the like, the aforementioned package, or the like, or damaging solder bonding between the heat spreader and the element or the like.
Examples of the Cu—Mo composite material constituting the Cu—Mo layer include:
(i) a Cu—Mo composite material prepared by heating a mixture of molybdenum (Mo) powder and Cu powder to the melting point of Cu or higher to melt the Cu and infiltrate the melted Cu into between Mo powder particles, and thereafter cooling and integrating them; and
(ii) a Cu—Mo composite material prepared by sintering Mo powder beforehand to fabricate a porous body (skeleton), infiltrating Cu into pores of such a porous body, and thereafter cooling and integrating them.
It should be noted that Cu is used together with Mo because the heat conductivity of Mo alone is insufficient and thus Cu is additionally used to prevent a decrease in the heat conductivity of the heat spreader.
As a heat spreader including such a Cu—Mo layer, a heat spreader made of a stacked body having a Cu layer stacked on each of both surfaces of the Cu—Mo layer has been proposed (see PTDs 1 to 3 and the like).
In order to satisfactorily solder-bond an element or the like to an element mounting surface of a plate-shaped heat spreader without generating voids and the like which interfere with heat conduction, it is preferable to form beforehand, on the element mounting surface, a nickel plating layer excellent in solder wettability and affinity.
However, since Cu and Mo, which are plated under significantly different conditions, are exposed in a surface of the Cu—Mo layer made of a Cu—Mo composite body, it is difficult to form a stable nickel plating layer directly on such a surface.
In contrast, since the heat spreader having a stacked structure described above has an element mounting surface made of Cu alone, a stable nickel plating layer can be formed thereon, and an element or the like can be satisfactorily solder-bonded on such a nickel plating layer without generating voids and the like which interfere with heat conduction. Further, since the Cu layer is excellent in solder wettability and affinity, it is also possible to omit a nickel plating layer.